I still have an iPod touch 2G on 4.2.1 (final) and I sold my iPod touch 4G on 4.3.5 a year or two ago after being annoyed by 6.1.x's slowness as I had SHSH blobs for everything 4.1 and up.
I also just bought a 3GS off Swappa for $30 that looked like it still had some version of 4.x installed as it still has the iPod app on the home screen. It'd be awesome if it was all the way on 4.3, but wouldn't surprise me if it was 4.1 as it's still a signed firmware for that device.
Of course I have many modern devices on 10.x and such, but I'm going to try and use this 3GS for a week or so to take a minute and unplug. EG, no social media or unnecessary apps, since most don't work on 4.x anyway.
I'll probably only install FB Messenger as the old simple version of it still works on my iPod touch 2G and I have people message on it all the time instead of SMS/iMessage. iMessage will be missed for a bit but I plan on turning if off a few days beforehand like I do when moving to Android. Or put it onto 6.1.6 if I must, but it's so much slower that way.
4.x was so fluid on it's native devices and even older 3GS/iPod touch 3G, but pretty horrid on the iPhone 3G. It's not half bad on the iPod touch 2G I have however, but it's clocked a bit higher than the 3G was.